The present study aims at the development of a reliable and efficient operation principle of the melting furnace for the combined treatment process of asbestos wastes with incineration ashes. A simple method to measure the molten slag fluidity was proposed, which can be applied on site. Three series of the verification tests using the actual ash-melting furnace of industrial solid wastes was carried out. It was confirmed that the proposed technology was effective to control the slag composition in the melting operation for actual wastes. It can treat safely and reliably asbestos wastes without residual asbestos in waste gas, slag, fly ash and working environment during the furnace operation. Further, it was confirmed that the formed slag was possible to recycle-use as construction materials and so on.